INT-4174: Expose Channel Subscriber Count

JIRA: https://jira.spring.io/browse/INT-4174

I considered just using the channel counter but I suppose there is a (small) possibility
that someone might possibly use a custom dispatcher and subscribe to it directly.

Polishing -- ** master only **

Eliminate conditional logic in AbstractMessageChannel by adding getHandlerCount() to
MessageDispatcher.

Do not cherry pick.

* Also fix JPA tests model for proper database `sequence` to avoid race condition when the test data is inserted with lower `id` than expected predefined data has

spring-integration-jmx/src/test/java/org/springframework/integration/jmx/MBeanAttributeFilterTests.java

@@ -120,6 +120,7 @@ public class MBeanAttributeFilterTests {
 				"MeanSendRate",
 				"MinSendDuration",
 				"StandardDeviationSendDuration",
+				"SubscriberCount",
 				"TimeSinceLastSend"));
 
 		adapterNot.stop();